In Published German Application No. 28 10 980 it has been proposed that, in an oil-cooled internal combustion engine in which oil is used to lubricate and to cool the engine, the cooling oil circuit should be in series with, and downstream of, the lubricating oil circuit, even though, in that case, the initial temperature of the cooling oil will be rather high and a high cooling oil flow rate will be required for adequate dissipation of heat. In addition, all of the cooling oil and lubricating oil will have to be recirculated through the oil cooler and the oil filter and this will involve considerable additional structural and power requirements. Moreover, carbonization of oil on the hottest portions of the engine may occur so that additional water cooling of such portions will be required.
Published German Application No. 35 09 055 discloses an oil-cooled internal combustion engine in which the lubricating oil circuit is in series and downstream of the cooling oil circuit so that warmed-up oil can be supplied to the lubricating points of the engine as the latter is warming up. But because that lubricating oil circuit branches off before the oil cooler of the cooling oil circuit, the temperature of the lubricating oil will remain substantially uncontrolled and a desirable cooling of high-temperature portions of the engine cannot be achieved.
From Published German Application No. 36 18 794 it is known to connect a lubricating oil circuit and a cooling oil circuit in parallel in an oil-cooled internal combustion engine. Each of the circuits is connected to a common oil sump and has a separate oil pump. The oil pumps discharge oil under different pressures. If the oil cooler is included in the lubricating oil circuit, the oil cooler will be subjected to the lubricating oil pressure and will cool the lubricating oil to a temperature which is much lower than the temperature of the oil sump; cooling oil from the oil sump will be supplied to the critical portions of the engine, such as the valve-carrying webs, the nozzle seats and the like and will be at an excessively high temperature. On the other hand, if the oil cooler is included in the cooling oil circuit, it will be subjected to a lower pressure and the critical portions of the engine can be cooled with relatively cool oil; however, the lubricating oil will be at the same temperature as the oil in the oil sump and can be cooled to a sufficiently low temperature only with a high cooling effort. In that case, high-temperature parts cannot adequately be cooled because the oil which is available for cooling the cylinder head will not be at a lower temperature than the oil in the oil sump and, in view of the critical points at the cylinder heads, the oil in the oil sump cannot be as cool as would be required for an adequate cooling of high-temperature portions. Whereas it is more desirable from a functional aspect to include the oil cooler in the lubricating oil circuit, the high pressure to which the oil cooler will then be subjected requires for the oil cooler an expensive design which is also unfavorable from the aspect of heat transfer.
A further disadvantage of the known oil-cooled internal combustion engines is that the oil level can be checked only with difficulty. Unless special means are provided, the cooling oil will gradually seep through the cooling oil pump and back into the oil sump when the engine is out of operation so that the oil level cannot be checked unless the engine has been out of operation for a rather long time. If the oil return passages contain special means, such as check valves, a return flow of oil before a check of the oil level will be prevented, but will also be prevented during an oil change, which is undesirable.
Published German Application No. 18 07 639 discloses a lubricating and cooling system which comprises a conventional engine-cooling system and in addition a special piston-cooling line, which branches from the lubricating oil circuit downstream of the oil cooler. This piston-cooling line will be opened only in dependence on the lubricating oil pressure and may be used only to supplement a conventional engine-cooling system so that the structural requirement will be increased.